In expanding the investigation of Ca++ regulation of microtubule assembly, we have studied the role of calmodulin. In the absence of microtubule-associated proteins (MAPs), calmodulin enhances polymerization by sequestering Ca++. In the presence of MAPs, calmodulin inhibits polymerization through binding to both MAP2 and 4 or 5 of the Tau protein as shown by affinity chromatography, equilibrium binding (Kd about 7 MuM) and cross-linking experiments, whereas tubulin does not react. We have also shown that subtle changes in tubulin that lead to a small reduction in Alpha-helix content have marked effects on the immunoreactivity of the protein with four different antibodies. These changes are reversible.